1. Field of the Invention
The present invention relates to a digital power amplifier, and for example, can be applied to an audio amplifier.
2. Description of the Related Art
Recently, digitalization of power amplifiers (amplifiers) is making quick progress. Particularly, in the audio amplifiers, digitalization is remarkably adopted in full scale. As equipment equipped with the digital power amplifier (a so-called switching amplifier), there can be mentioned DVD players, minicomponents, television sets, personal computers, portable telephones and so forth.
FIG. 7 is a block diagram showing the configuration of the vicinity of a load (speaker) in a conventional switching amplifier having positive and negative two power sources.
Switching elements FET1 and FET2 including, for example, FET are connected between power supply lines +B and −B of the positive and negative two power sources of a power supply section formed of a transformer T, diodes D1 and D2, and power supply capacitors C1 and C2. A coil L and a capacitor C constituting a low-pass filter are connected between a node between the switching elements FET1 and FET2 and a node between the power supply capacitors C1 and C2, and a speaker SP as a load is connected in parallel with the capacitor C constituting the low-pass filter.
The pair of switching elements FET1 and FET2 is basically on-off controlled complementarily, corresponding to one-bit digital signal (PWM signal) obtained by converting an analog audio signal. FIG. 7 shows a state in which the switching element FET1 is switched on. By the ON action of either one of the switching elements FET1 and FET2, power supply current flows in the forward direction or the reverse direction with respect to the capacitor C constituting the low-pass filter. As a result, appropriately changed voltages across the capacitor C (voltage obtained by amplifying the initial input analog audio signal) are applied to the load SP.
Each of the both ends of the low-pass filter coil L is fed back to an analog amplifier section (not shown), which amplifies the input analog audio signal, via feedback resistances R1 and R2, to stabilize the output of the low-pass filter.
When the switching element, which has been switched on, is to be switched, through-current flows over the both switching elements, due to a difference between the transition characteristic from on to off and the transition characteristic from off to on, of the switching elements.
In order to prevent this, there is heretofore provided dead time, during which the both switching elements are switched off, by setting the timing for switching off the switching element, which has been switched on, slightly earlier than the timing for switching on the switching element, which has been switched off.
An unnecessary high pass is removed by the low-pass filter, to contemplate stabilization by the feedback function. In the case of the switching amplifier, however, there is a problem in that at the time of a high impedance load or no load, a peak occurs in the high pass in the vicinity of the cut-off frequency of the low-pass filter, as shown in the frequency characteristic diagram in FIG. 8.
Therefore, as shown in FIG. 9, it can be considered to provide a damper including a capacitor CD and a resistance RD in parallel with the low-pass filter capacitor C. However, a new problem occurs in that a power loss occurs in the damper.
Moreover, as described above, dead time is provided so that the pair of switching elements FET1 and FET2 does not switch on at the same time (so that the through-current does not flow).
However, when an MOSFET is employed as the switching element, it includes a built-in diode (body diode), and hence the reverse recovery is very slow. Therefore, when switching is carried out at a high speed, the switching element cannot be switched off instantaneously, and even if the switching element itself is switched off, reverse current flows to the built-in diode due to a carrier storage effect. As a result, through-current flows from the switching element, which is switched on, to the switching element, which should have been switched off.
As a method of preventing the through-current resulting from the delay in the operation of the built-in diode, as shown in FIG. 10, there is used a method in which high-speed diodes D3 and D4 for blocking reverse current are provided in series with the switching elements, and high-speed diodes D5 and D6 for bypassing counter-electromotive voltage are provided in parallel with the switching elements. The high-speed diodes D5 and D6 for bypassing counter-electromotive voltage form a path in which energy accumulated, when the power supply current flows, in the low-pass filter coil L flows as the current due to the counter-electromotive voltage, immediately after the switching element, which has been switched on, is switched off.
However, if the high-speed diodes D3 and D4 for blocking reverse current are provided, there is a problem in that a voltage loss occurs due to the high-speed diodes D3 and D4, thereby decreasing the efficiency.
Therefore, a digital power amplifier having high power amplifying efficiency has been heretofore desired.